The constancy of the diameter of an optical fiber is important for the minimization of losses in the transmission of signals therethrough. In order to insure such constancy, the fiber should be continuously monitored during its manufacture, i.e. while it is being drawn from a heated preform at a speed which can be adjusted to minimize deviations from a predetermined diameter. Such a monitoring operation requires a precise measurement of the fiber diameter for the purpose of detecting any incipient change.
A conventional method of measuring that diameter utilizes a laser beam which is reciprocated across the path of the fiber in a plane perpendicular thereto and illuminates a photodiode. With that method it is possible to calculate the fiber diameter from the known speed of the angular beam motion and from the interval during which the photodiode is obscured. The equipment required for this purpose is rather expensive and, with present-day facilities, has a resolution on the order of one micron which often is insufficient.
Another method involves the focusing of light from a fixed source upon a photodiode matrix, with the aid of an optical system such as a microscope objective, across the fiber path and measuring the extent of the shadow cast upon that matrix. This technique is suitable for fibers traveling with a precisely controlled motion but is unsatisfactory for monitoring during the fiber-drawing process since unavoidable vibrations of the fiber tend to cause major errors detectable only by a repetition of the measuring operation. Such oscillations of the advancing fiber also make it difficult to measure the diameter by interferential methods since the continuous shifting of the interference fringes impedes their evaluation.